The bookshelves that line one wall of Senator William “Mo” Cowan’s office are empty. Former senator and now Secretary of State John Kerry once occupied this same elegant space in the Russell Senate Office Building. He left, taking all of his books and memorabilia with him, and Cowan arrived, bringing very little. There was no reason to: He knew his time would be short.

Cowan was sworn in on Feb. 7, and he’s scheduled to leave by the end of June. Five months in the Senate isn’t enough time to accomplish much, and people in Cowan’s position — temporary appointments to the job — are derisively called “seat warmers.” They’re there to take votes and make sure the state they represent isn’t forgotten, and on those fronts, Cowan has acquitted himself well. But think of this too as the grown-up equivalent of a college student’s semester abroad. Or, perhaps, Mo Cowan’s excellent adventure.

The day we meet, one top staffer, inherited from Kerry, is leaving; Cowan’s days in the nation’s capital are winding down. “It’s been exhilarating,” he says, “the thrill of my life.” He is the seventh African-American ever sworn in as senator. Raised in rural North Carolina (he recalls Ku Klux Klan members burning crosses and marching on his high school), he made his way to Massachusetts via Northeastern University’s School of Law. He’s only 44, and his career has been meteoric: partner at the influential law firm of Mintz Levin, legal counsel to Governor Deval Patrick, governor’s chief of staff. His wife and two young children live in Stoughton, and he commutes, flying to D.C. on Mondays and heading home Fridays. “The toughest part,” he says, “is being away from my family.”

Cowan’s role as a placeholder makes him, he says, “either the most irrelevant or the most dangerous” person in the Senate. The Senate cares a great deal about seniority and Cowan is at the bottom of the list. On the other hand, he says, “I’m not constrained by dollars or donors.” In a city where job one is running for reelection, Cowan has an unusual degree of freedom. He’s had the time to become a student of the legislative process. He’s met other congressional members and staff, developed relationships around town, and been able to think about what works and what does not.

He acknowledges the partisanship that now grips the Capitol. Yet the Senate, he says, “is truly a club.” Every member — Democrat or Republican — “has been warm and welcoming.”

“Outside of votes,” he says with a smile, “true collegiality.”

There have been moments of disappointment, such as the failure to pass gun background checks. And he laments the effect of the Senate’s 60-vote procedural rules, which often mean a super-majority is needed to pass legislation. “Majority rule ought to mean something.”

He contrasts the slow-moving Senate with its long time horizon to the rapid-fire decision-making he saw in the governor’s office. He observes the differing political pressures politicians from around the country face, the way that votes on the Senate floor really are dictated by politics at home. Senators from the swing states — “the purple states” — fascinate him for their unpredictability and the opportunities they have to reach across aisles. (The better politicians, perhaps, are the ciphers.)

Still, for all of the perceived faults of Washington, he disagrees with those who claim the system is broken. “The solution is elections,” Cowan argues. Things will change “when people decide they want to work together.”

Cowan, a classic inside player, was largely unknown to the public when Governor Patrick first named him. Now with a higher profile — he’s spent much time in office traveling the state — he insists the next step is not political office: “I love serving, but I think I’d loathe running.”

Perhaps. There is wistfulness is his voice as Cowan talks about leaving the Senate. He’ll be back in Massachusetts just in time for summer and plans to spend a few months relaxing and playing with his kids. He says he might rejoin the private sector. From there, he could easily fade away, getting well-paid by clients eager for his smarts and connections. But Cowan doesn’t strike me as the type inclined to waste adventures. Expect more from him. Much more.

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